1. Field of the Invention
The present invention relates to a tire abnormality sensor for sensing abnormality of tires of a vehicle.
2. Description of Related Art
A tire abnormality sensor is an apparatus for monitoring the state of tires during drive, sensing abnormality such as deflation, and notifying a driver or the like of the abnormality. As a technique of sensing the tire state, other than a conventionally-known technique of sensing tire pressure or temperature in a tire, there is a technique in which attention is paid to the fact that the tire radius is changed when the tire is deflated, and an angular velocity of a wheel is sensed. As a method using such a technique of sensing the tire state, Japanese Patent Laid-Open Publication No. Hei 4-232107 discloses a method of sensing the deflation of a tire by calculating a linear function of the difference between the squares of the angular velocities of a pair of first and second wheels.
Hitherto, a rotational speed sensing mechanism for sensing speed of a rotor has a signal rotor which rotates integrally with the rotor. A number of pulse signals are successively generated from electromagnetic pickups or the like in association with the rotation. An electronic control unit (ECU) receives the pulse signals and calculates the speed of the rotor on the basis of the number of pulse signals per unit time or pulse signal periods. However, the pulse signals are not constant due to nonstandard factors such as manufacturing error, corrosion, deformation during drive or the like of the signal rotor. It is, therefore, necessary to correct the sensing error of each pulse signal which is caused by the nonstandard factors. Japanese Patent Publication Laid-Open No. Hei 6-308139 discloses a rotor speed sensor in which the sensing error of the pulse signal due to the nonstandard factors is corrected. According to the rotor speed sensor, the periods of the pulse signals of the newest rotation are stored, the average of the pulse signal periods is sequentially calculated on the basis of the stored pulse signal periods, and a correction coefficient for correcting the pulse signal period is updated by using the average as a learning reference value, thereby eliminating the sensing errors due to the nonstandard factor.
According to the tire deflation sensing method of Japanese Patent Publication Laid-Open No. Hei 4-232107, however, only whether the tire is deflated can be determined from the linear functions obtained by inputting the angular velocities of the four wheels. Detailed information on the tire in which abnormality occurred, for example, whether the deflated tire is on the front or rear side cannot be obtained.
According to the rotor speed sensor disclosed by Hei 6-308139, although the sensing error caused by the nonstandard factor can be almost eliminated, since calculation scale in the ECU is large, an ECU having high processing ability is required. Consequently, an ECU having large memory capacity or which can perform a high-speed process is necessary.